doc: Add note about possible performance overhead by enabling track_planning.

Enabling pg_stat_statements.track_plaanning may incur a noticeable
performance penalty, especially when a fewer kinds of queries are executed
on many concurrent connections. This commit documents this note.

Back-patch to v13 where pg_stat_statements.track_plaanning was added.

Suggested-by: Pavel Stehule
Author: Fujii Masao
Reviewed-by: Pavel Stehule
Discussion: https://postgr.es/m/CAFj8pRC9Jxa8r5i0TNBWLb8mzuaYzEoLq3QOvip0jVpHPOLbVA@mail.gmail.com

doc/src/sgml/pgstatstatements.sgml

@@ -607,6 +607,9 @@
      <para>
       <varname>pg_stat_statements.track_planning</varname> controls whether
       planning operations and duration are tracked by the module.
+      Enabling this parameter may incur a noticeable performance penalty,
+      especially when a fewer kinds of queries are executed on many
+      concurrent connections.
       The default value is <literal>off</literal>.
       Only superusers can change this setting.
      </para>